1. Field of the Invention
This invention relates to power transmission apparatuses and, more particularly, to a power transmission apparatus in which a unit motor is used to drive a plurality of driving systems in changing-over.
2. Related Art and Prior Art Statement
Conventionally, various kinds of drivingforce changing-over or switching mechanisms in which unit driving force is used drive a plurality of driving systems in changing-over have been proposed in equipment such as a camera or the like. For example, a driving-force changing-over mechanism for changing over driving force of a unit motor to a plurality of transmission mechanisms by normal and reverse rotational operation is disclosed in Japanese Patent Laid-Open No. HEI 1-287648.
This technical means is so arranged as to change over the motor such that a driving force of the unit motor is utilized as a driving source such as shutter charge, mirror driving and the like by operation in normal rotation of the technical means, and driving force of the motor is utilized as driving force for winding operation of a film and rewinding operation of the film by reverse rotational operation.
Further, technical means (first technical means) for performing clutch changing-over operation by rotation in one direction by a rotary clutch, and a non-driving mechanism is driven by rotation in the other direction has been proposed in Japanese Patent Laid-Open No. HEI 5-321986.
By the way, in such changing-over mechanism, positional detection of a changing-over element becomes an indispensable technique. However, in Japanese Patent Laid-Open No. HEI 5-321986, technical means (second technical means) has been proposed in which an initial position is detected by a length of a changing-over range of a cam which lifts a lever.
Moreover, the inventors of the present application have proposed technical means in Japanese Patent Application No. HEI 4-295139, in which portions different in reflectance or reflection factor from each other are provided on a portion of a cam which has a plurality of engaging positions, to thereby enable detection of an absolute position.
Meanwhile, a driving mechanism has been proposed in Japanese Patent Application No. HEI 3-309336, in which a unit motor is used to select a plurality of gears to be driven by combination of a sun gear and a planetary gear.
Furthermore, the inventors of the present application have proposed a changing-over mechanism in Japanese Patent Application No. HEI 4-268878, for detecting a resetting position from a pulse width of a signal during driving.
However, a complicated mechanism is required for the technical means disclosed in Japanese Patent Laid-Open No. HEI 1-287648, because an engaging element of the planetary gear is changed over by a section changing-over element. This makes it difficult to down-size or miniaturize equipment such as a camera or the like, and causes an increase in cost. Further, the first technical means which has been proposed in Japanese Patent Laid-Open No. HEI 5-321986 is difficult to detect an absolute position of a rotary clutch section.
Moreover, it is impossible for the second technical means proposed in Japanese Patent Laid-Open No. HEI 5-321986 to perform positional detection only in a case where an engaging mechanism moves at equal or constant speed. Furthermore, the second technical means cannot also detect an absolute position.
Further, such a deficiency occurs in the technical means proposed in Japanese Patent Application No. HEI 4-295139 that a sensor must be arranged in the vicinity of the cam so that layout is limited, and that, in a case of miniaturization, a detecting error increases so that it is impossible to make a section to be detected small.
Meanwhile, the technical means which has been proposed in Japanese Patent Application No. HEI 3-309336 is such that a series of release operations are performed after an initial gear of a driving system for driving an auto-focus lens has been selected, at the time of release operation. Therefore, release time lag occurs. Thus, the technical means causes deterioration of a feeling of operation.
Moreover, technical means which has been proposed in Japanese Patent Application No. HEI 4-268878 is not provided with means for detecting the initial position. Accordingly, once the initial position is mistaken by erroneous operation or the like, there is danger that the operation continues under this state. Furthermore, there is also danger that, under a stationary state, detection of the position becomes difficult.